1. Field of the Invention
The present invention relates to a method for the combustion of coal wherein the emissions of SO.sub.2 and NO.sub.x are reduced by retention of sulfur compounds in the slag and for the reduction of fuel nitrogen compounds to molecular nitrogen in the combustion chamber.
2. Description of the Prior Art
In the combustion of carbonaceous materials such as coal which contain sulfur and ash, oxygen combines with the sulfur to produce sulfur dioxide. Production of sulfur dioxide is undesirable and emission of sulfur dioxide is limited by government regulation for power plants built after 1972 and may soon be limited for power plants built before 1972. To comply with these regulations, utilities may elect to use naturally occurring low sulfur coals, clean high sulfur coals to reduce the sulfur content, substitute low sulfur oil or natural gas, inject calcium based sorbents into the boiler, or use calcium, sodium, or magnesium based solutions to scrub sulfur dioxode from the stack gases. All these techniques are expensive and some cannot be easily retrofitted to existing plants. As a result, attempts have been made to develop a process to burn high sulfur coal without increasing the emissions of sulfur dioxide.
In U.S. Pat. No. 4,232,615, Brown teaches that sulfur can be captured as a sulfide under rich conditions. Lyon et al. in U.S. Pat. No. 4,285,283 teach that sulfur can be captured in the slag and separated as water soluble sulfides at fuel equivalence ratios greater than 1.5 and temperatures between 1200.degree. and 1400.degree. C. (2190.degree. and 2550.degree. F.). In U.S. Pat. No. 4,407,206, Bartok et al. teach that sulfur capture in combustion of coal can be as high as 70 percent at a fuel equivalence ratio of 1.5 and 80-95 percent combustion of the coal at 1200.degree.-1300.degree. C. (2190.degree.-2370.degree. F.). A maximum of 35 percent sulfur retention was achieved at similar conditions and 1400.degree. C. (2550.degree. F.).
In the article "A Review of In-Furnace NO.sub.x. Reduction and SO.sub.x Reduction by Sorbent Injection", EPRI CS-1382, Breen et al. teach that retention of sulfur in solids or liquids is favored by substoichiometric conditions and decreases with increasing temperature.
England et al. in their article "Coal-Fired Precombustions for Simultaneous NO.sub.x, SO.sub.x and Particulate Control", EPA/EPRI Primary Source NO.sub.x Symposium, Boston, Mass., May, 1985, teach that retention of sulfur as sulfide under rich conditions is favored over retentions of sulfur as sulfate under lean conditions at high temperatures.
Finally, in U.S. Pat. No. 4,523,532, Moriarty et al. teach that sulfur can be retained in the slag under conditions of low excess air.
None of the above processes have been brought to commercial success because of the problems in applying the technology to existing combustors, problems retaining the sulfur in the slag and achieving good burnout, the potential of interrupted slag flow, and the potential of increased tube wastage. Therefore, a need exists for an inexpensive process, retrofitable to existing boilers, to remove sulfur and reduce nitrogen oxides without aggravating operation problems of the boilers.